1. Field of the Invention
The present invention relates generally to composite electrodes and more particularly to a composite carbon electrode useful for electric double layer capacitors and/or for capacitive deionization and methods of making the same.
2. Technical Background
An electric double layer capacitor (EDLC) is an example of a capacitor that typically contains porous carbon electrodes (separated via a porous separator), current collectors and an electrolyte solution. When electric potential is applied to an EDLC cell, ionic current flows due to the attraction of anions to the positive electrode and cations to the negative electrode. Electric charge is stored in the electric double layer (EDL) formed along the interface between each polarized electrode and the electrolyte solution.
EDLC designs vary depending on application and can include, for example, standard jelly roll designs, prismatic designs, honeycomb designs, hybrid designs or other designs known in the art. The energy density and the specific power of an EDLC can be affected by the properties thereof, including the electrode and the electrolyte utilized. With respect to the electrode, high surface area carbons, carbon nanotubes, activated carbon and other forms of carbon and composites have been utilized in manufacturing such devices. Of these, carbon based electrodes are used in commercially available devices.
Capacitive Deionization (CDI) is a promising technology, for instance, for the purification of water. In this context, positively and negatively charged electrodes are used to attract ions from a stream or bath of fluid. The ions form electric double layers on the surfaces of the electrodes, which are fabricated from some form of high surface area material, for example, a form of activated carbon. After interaction with the electrodes during the charging period, the fluid contains a lower overall ion content and is discharged. A volume of purge fluid is then introduced to the electrodes. The electrodes are then electrically discharged, thus releasing the trapped ions into the purge fluid. The purge fluid is then diverted into a waste stream and the process repeated.
In U.S. Pat. No. 6,778,378, electrodes are described which are fabricated by blending activated carbon powder, fibrillated polytetrafluoroethylene (PTFE) and a liquid, forming a paste-like material. The resulting paste-like material is then rolled into a composite sheet. Electrodes formed in this fashion are thin, flexible sheets with good specific capacitance. A large fraction of this capacitance value is a result of the activated carbon used.
However, the particle to particle connectivity of the carbon particles in the activated carbon is typically poor and these electrodes have high electronic resistance compared to the monolithic aerogel electrodes. A separate current collector sheet, typically made of rolled exfoliated graphite material, is clamped to the electrode back surface with a large compressive force to obtain the necessary electrical performance, thus increasing the cost and the complexity of a CDI device made using these electrodes. Also these rolled composite sheets, due to the purely mechanical nature of the PTFE/carbon bonding, have only modest erosion resistance. For this reason, when eliminating particulates from the fluid stream, the flow rates of the fluid stream need to be minimized across the surface of these electrodes.
In U.S. Pat. No. 5,636,437, CDI electrodes are described which are fabricated from a slurry of carbon powder in a polymer binder. High surface area carbon powder is mixed with a carbonizable polymer liquid to form a paste-like material. The resulting paste-like material is then applied to an electrically conductive porous substrate. The polymer liquid is cured and carbonized, creating an all-carbon electrode where the high surface area particles are embedded within the carbonized polymer. The utility of the activated carbon powder is compromised in this design as a result of the activated carbon powder being embedded within the carbonized polymer matrix.
In U.S. Pat. No. 5,776,633, electrodes are described in which discrete activated carbon fibers or a piece of activated carbon fiber cloth is coated with a mixture of carbonizable polymer and an activated carbon powder. The carbonizable polymer is cured and carbonized, creating an all-carbon electrode where the high surface area activated carbon particles are embedded within the carbonized polymer. The utility of the activated carbon powder is compromised in this design as a result of being embedded within the carbonized polymer matrix.
In U.S. Pat. No. 5,932,185, electrodes are described which are fabricated by coating carbon paper with a carbon foam. Carbon paper sheets are soaked with a polymer resin sol and then pyrolyzed. Supercritical drying is traditionally used to obtain an aerogel structure prior to pyrolysis. The resulting electrodes are thin, monolithic carbon structures. The aerogel surface of these electrode sheets and the carbon paper itself are delicate and need to be protected from mechanical stressing. The resulting electrodes exhibit good CDI performance, but are extremely costly. Limited success has been achieved at applying subcritical drying to reduce cost. Also, these electrodes possess a very modest level of total capacitance per unit area, as the aerogel layer is thin and of limited surface area. This increases the number of electrode sheets required for a given system, further increasing the cost.
A flow-through (rather than parallel plate) flow geometry is described in commonly owned U.S. Pat. No. 6,214,204, the disclosure of which is incorporated herein by reference in its entirety. In this reference, monolithic, low back pressure porous electrodes are made by one of several methods, which include honeycomb extrusion, casting or molding from a phenolic resin-based batch. After curing, these parts are carbonized and activated to create high surface area carbon monoliths with good electrical conductivity.
The resulting electrodes may exhibit limited mechanical strength, in certain embodiments, since the electrodes may comprise a porous and brittle material. Thus, manufacturing thin, large diameter electrodes for high performance is challenging and packaging the electrodes into a CDI system is also challenging. Also, because they do not have a conductive graphitic backplane, their electronic conductivity is low as compared to the carbon paper-based aerogel electrodes.
Commonly owned U.S. patent application Ser. No. 11/799,901, the disclosure of which is incorporated by reference in its entirety, describes layered carbon electrodes for use in, for example, Capacitive Deionization (CDI) of a fluid stream or, for example, an electric double layer capacitor (EDLC). Methods of making the layered carbon electrodes are also described. The layered carbon electrode comprises an electrically conductive porous layer and an adjacent layer comprising carbon particles in contact with the electrically conductive porous layer. A thermoplastic material is infused in the electrically conductive porous layer and provides a bond to the carbon particles at the interface of the electrically conductive porous layer and the adjacent layer comprising carbon particles.
Commonly owned U.S. patent application Ser. No. 11/807,981, the disclosure of which is incorporated by reference in its entirety, describes carbon electrodes for use in, for example, Capacitive Deionization (CDI) of a fluid stream or, for example, an electric double layer capacitor (EDLC). Methods of making the carbon electrodes are also described. The carbon electrode comprises an electrically conductive porous carbon support and a carbon cover layer comprising carbon particles in contact with the electrically conductive porous carbon support. A carbonizable material is within the electrically conductive porous carbon support and provides a bond to the carbon particles at the interface of the electrically conductive porous carbon support and the carbon cover layer. The electrically conductive porous support in some embodiments is a layered structure, where one of the layers is a carbonizable paste layer having electrically conductive particles mixed therein.
The layered carbon electrodes, described above, have increased electrical conductivity as compared to many conventional electrodes, in part, due to the integrated electrical backplane provided by the electrically conductive porous layer. The layered carbon electrodes possess increased capacitive performance, resulting from highly effective utilization of the carbon particles.
U.S. Pat. No. 5,443,859 describes a method of manufacturing high conductivity carbonized films made from a polyimide.
Several types of photoresists are described in the Journal of Electrochemical Society, 152 (12), pp. J136-J143 (2005) for the manufacturing of carbonized films with high conductivity. A process for the manufacturing of carbonized films from photoresists such as PMMA and SU-8 is described for micromechanical systems.
It would be advantageous to develop cost-effective, electrochemically inert and mechanically robust carbon electrodes, including increased erosion resistance, with high specific capacitance and low electrical resistance. Also, it would be advantageous for the carbon electrodes to be easily processed into different geometries which could enable various fluidic schemes.